It's hard to give you solid advice without too much context but if they were your closest friend and you really didn't want to cut them out of your life, some things are worth fighting for or else you feel the exactly way you do now. I don't know what they could've said to you to instantly not be your friend anymore, but it must've been bad to cut a potentially lifelong friend out. Did they apologize to you? Were they genuine about it if they did?

When it comes to best friends sometimes what you should do isn't always the right choice. Friends that are really close to eachother tend to push the line of what is acceptable because they know what bothers you the most. But they also know you best and if you arent seeing eye to eye on something arguments can get out of hand quickly.

Also are you sure they are doing fine without you? Or are you just assuming they are because you haven't talked to them? Friend breakups are the worst and leave lasting scars on both sides. Its possible that they are good at hiding them.

I know right know it's still fresh and painful, and you may realize one day that you either made the right choice or you want them back in your life. Just do whatever you believe is right. Make sure you have no regrets because it will eat you alive.